1. Field of the Invention
The present invention relates to an apparatus for compression-encoding and decoding digital video signals, and more particularly to an apparatus for compression-encoding and decoding digital video signals, which intends to recover fine information disappearing upon interpolating a digital video compressed by a decimation and to enable a circuit at the receiving side to be efficiently designed.
2. Description of the Prior Art
Generally, compression of digital video signals means that a great quantity of video data is efficiently converted into a small quantity of video data to meet the recording bandwidth of a recording medium or a channel, although there may be a degradation in picture quality of an original video.
As conventional video compression techniques, various coding methods have been known, which include a transform coding method, a predictive coding method, a hybrid coding method, a vector quantization method, and a sub-band coding method. Recently, such video compression techniques have been making fast progress. In particular, as they have been applicable to technical fields such as ISDNs and multimedia, international standards relating to the video compression have been continuously reported.
For example, the international standard so called a moving picture experts group (MPEG) for recording a compressed video on a digital recording medium such as a CD-ROM is currently extending.
In accordance with this MPEG standard, it is possible to achieve an encoding at a high compression ratio, using methods such as a discrete cosine transform (DCT), a motion estimation (ME) and a variable length coding (VLC).
Referring to FIG. 1, there is illustrated a conventional device for compression-encoding video signals. As shown in FIG. 1, the compression encoding device comprises a decimation unit 1 for reducing the amount of data for an original video in a sub-sampling manner, a first encoding unit 2 for encoding output data from the decimation unit 1 to compress it, and a VLC/formatter 3 for compression-formatting an encoded signal from the first encoding unit 2 in a VLC manner.
On the other hand, FIG. 2 illustrates a conventional device for decoding compressed video signals. This decoding device comprises a decoder 4 for receiving compression-encoded video data and decoding it, and an expand unit 5 for expanding video data decoded in the decoder 1.
Operations of these devices for compression-encoding video signals and decoding them will now be described.
As an original digital video signal is received in the compression encoding device, the amount of data thereof is reduced in the decimation unit 1, in a sub-banding coding manner. Thereafter, the video data reduced in data amount is encoded in the first encoding unit 2 to be compressed. The compression-encoded signal is then formatted in the VLC/formatter 3, in a variable length coding manner, so as to be compressed at a higher rate. This compressed video data is recorded on a recording medium or transmitted via a transmitting medium.
On the other hand, when a signal obtained by reproducing the signal transmitted from the video signal compression encoding device or recorded on the recording medium is received in the decoding device, it is decoded in the decoder 4 and then expanded to the size of the original video signal, in the expand unit 5, so as to be recovered.
The first encoding unit 1 utilizes high technical video compression techniques, so as to transmit video data with a proper size and a proper amount within a certain limited bandwidth. For example, video compression techniques which are regarded as optimum ones for respective channels are selected in various fields such as HDTV fields, digital VCR fields, and multimedia fields.
Where a higher compression ratio is desired, in spite of a degradation in picture quality, a device such as the decimation unit 1 is used. In this case, there is used a method for reducing a video in size in the sub-sampling manner and then compressing it. When this method is used, fine information of the video disappear during when the size of video information is optionally reduced.
In cases of video data compression methods used in HDTV digital VCRs and digital broadcasting satellites (DBSs), the use of the decimation unit 1 is eliminated so that an original video subjected to no sub-sampling is compression encoded, in order to generate a video signal with a certain limited bandwidth. In this case, there are various problems relating to the data amount.
Each of the above-mentioned video data compression techniques are designed for one selected channel or recording medium. If a new application requiring a larger bandwidth is made, an optimum video data compression technique will be newly developed. However, such a video data compression technique newly developed may have no relation with the known video data compression techniques. In this case, there is a problem about a compatibility to systems using the known compression techniques. For example, it is impossible to utilize the new compression technique in systems using the known compression techniques, since a new decoding technique should be used, in order to decode a video signal encoded according to the new compression technique. Furthermore, a new system to which a new decoding technique is applied can not decode video data encoded according to the known compression techniques.
Due to such a problem about compatibility, the manufacturers need a long time for developing new systems. On the other hand, the consumers bear expenses for purchasing the systems. As a result, there is also a problem of requiring a long system replacement time for enabling a commercial use of more efficient video processing systems meeting new and larger bandwidths.
Until an optimum video compression technique is developed, it is necessary to provide compression encoding/decoding devices capable of being applied to new bandwidths and yet minimizing an alternation of existing systems, even though they exhibit a slight degradation in picture quality.